The present invention relates generally to RF transmitters, and more particularly, to improved high power RF transmitters employing high power QAM modulators.
For very high data rate and bandwidth efficient transmission, quadrature amplitude modulation (QAM) has become a major waveform option. Many modulator systems have been developed to perform quadrature amplitude modulation and compensate for bandpass distortion and nonlinear effects so that very high symbol rates (in excess of 1 Gsps) may be achieved. Conventional data modulation is performed at low power levels and sometimes at an intermediate frequency, and then frequency shifted and amplified to a suitable power level.
More recently, the assignee of the present invention has developed a modulating array transmitter which is disclosed in U.S. Pat. No. 5,612,651. This patent application teaches a method of combining the outputs of saturated power amplifier stages where each amplifier stage is driven by a QPSK modulator. The amplifier output signals are combined to form QAM high power signals. The invention disclosed in this patent application teaches that locating the modulation closer to the transmitting antenna reduces the number of distorting and performance-limiting elements of the transmitter.
Several patents were uncovered as a result of a computer search performed during the development of the present invention. U.S. Pat. No. 4,453,257 entitled "Spectrum Shaping Microwave Digital Modulators" spectral shaping of data NRZ (non-return to zero) waveforms with combined linear phase modulation methods as a substitute for phase keying modulation. This patent is not relevant to the present invention.
U.S. Pat. No. 4,835,493 entitled "Very Wideband Linear Amplitude Modulation of RF Signal by Vector Summation" discloses the use of linear phase modulators followed by high power amplifiers, but does not discuss the basic vector combining application to high rate QAM digital modulation. The focus of this patent relates to amplitude modulation wherein linear phase modulated signal components are used to achieve only linear amplitude modulation.
U.S. Pat. No. 5,329,259 entitled "Efficient Amplitude/Phase Modulation Amplifier" discloses a means of creating QAM signals by using two constant amplitude vectors. This patent discusses the use of QAM waveforms. However, this patent is not specifically relevant to the present invention.
Accordingly, it is an objective of the present invention to provide for improved high power RF transmitters employing high power QAM modulators.